This invention relates to lasers and, more particularly, to quantum wire (QWR) lasers operating in the quantum limit.
It is well known that quantum well (QWL) semiconductor lasers exhibit improved performance characteristics compared to conventional heterostructure diode lasers. In QWL lasers, charge carriers are quantum confined in one dimension in extremely thin active layers. Additionally, it is known that further improvement in semiconductor laser performance can be expected if quantum confinement can be achieved in more than one dimension.
In an article entitled "Quantum Wire Lasers", by E. Kapon, Proceedings of the IEEE, Vol. 80, No. 3, Mar. 1992, pages 398-410, there are described quasi one-dimensional structures in which optical gain is provided by charge carriers that are quantum-mechanically confined in two dimensions within wire-like active regions. Such QWR lasers are predicted to exhibit lower threshold currents, higher modulation bandwidths, narrower spectral linewidths and reduced temperature sensitivity compared to their QWL counterparts.
In practice, however, it has become evident that the actual fabrication of quantum-size structures with precise control in more than one dimension is an extremely challenging task. Several techniques for making such structures have been reported. Some of these techniques suffer from problems such as size fluctuations, interfacial disorder and the introduction of nonradiative recombination centers which wipe out quantum effects stemming from reduced dimensionality. One reported technique [see E. Kapon et al., Phys. Rev. Lett. 63, 430 (1989)] has been demonstrated to be a powerful one for fabricating quasi quantum wires exhibiting some characteristics of two-dimensional carrier confinement. But, as a consequence of the relatively large size of the wire-like structure achieved thereby, many one-dimensional subbands exist and, due to band-filling effects, only higher-order transitions can be observed in the stimulated emission spectra. Since the density-of-states spectrum in this energetic region already approaches that of the two-dimensional case, no improvement in threshold current over conventional QWL lasers was reported.
Accordingly, efforts have continued by workers skilled in the art directed at trying to provide improved QWR structures. In particular, these efforts have been directed at trying to develop a QWR structure having an extremely small cross-section that could in practice be fabricated to exhibit confinement in the one-dimensional quantum limit, i.e. in QWR structures with sufficient carrier confinement in two dimensions to allow for the formation of only a single bound one-dimensional state. It was recognized that these efforts, if successful, could provide a practical QWR laser characterized by a threshold for stimulated emission that is substantially reduced compared to that of a QWL structure.